The present invention relates to methods and apparatus for roasting coffee and similar particulate vegetable materials.
Coffee beans are roasted to develop the characteristic flavor and aroma of the product as used by consumers. The flavor and aroma of green coffee are not desirable; but when green coffee beans are roasted, complex, thermally-induced chemical reactions convert compounds contained in the beans, such as sugars, amino acids, polysaccharides, proteins, trigonelline, chlorogenic acids, and others into more than 800 compounds that collectively provide the desirable, extractible flavor, color and aroma characteristic of roasted coffee. Some green coffee components that do not react during roasting, such as caffeine, contribute to roasted coffee""s stimulatory action and flavor, but most of roasted coffee""s flavor, color and aroma is provided by compounds generated in roasting-induced reactions.
Coffee roasting involves systems of interdependent chemical reactions that proceed along series and parallel reaction paths. Rates of these reactions increase markedly, but to different extents, as bean temperature increases. Because of reaction interdependence and the varying effects of temperature on individual reaction rates, the makeup and yield of products generated by roasting depend on bean temperature versus time history during roasting. Consequently, the flavor and aroma of roasted coffee depend on that history. Control of the temperature-versus-time history of the coffee during the roasting process would greatly enhance control of flavor and aroma.
Roasting initially is endothermic; i.e. heat transferred to coffee beans raises their sensible heat content, evaporates water and provides heat used in endothermic reactions. After bean temperatures reach 160xc2x0 C., rapid exothermic reactions occur, bean temperatures rapidly rise and coffee""s flavor changes very rapidly. Excessive weight loss and undesirable flavor changes occur if roasting is excessively prolonged. Therefore, to end roasting quickly and provide coffee of desired, reliably duplicated quality, beans most commonly are rapidly cooled (quenched) as soon as they reach a selected end-of-roast temperature. First, a controlled amount of water, is sprayed on the beans and largely evaporates, providing evaporative cooling. Then, the beans are cooled further by forced contact with ambient-temperature air.
Reflectance color is the fraction of incident light of selected spectral composition that is diffusely reflected from the surface of a suitable sample of compressed, ground, roasted coffee. The lower the reflectance color, the darker the coffee. End-of-roast temperatures correlate well with roast darkness, as measured by reflectance color. Roast darkness, in turn, roughly correlates with flavor. Some consumers prefer relatively dark and bitter roasted coffees; others prefer relatively light, somewhat acid coffees; and still others prefer coffee of intermediate character.
Roasting conditions also influence the bulk density (mass of coffee per unit volume) of the roasted coffee beans. As further explained below, under certain roasting conditions the coffee beans can be xe2x80x9cpuffedxe2x80x9d by internal pressure of steam and other gasses when the walls of the beans soften at elevated temperature. Such puffing reduces the bulk density. The bulk density of the roasted beans in turn influences the bulk density of the ground product as sold to the consumer and the weight of coffee which fits into a standard coffee can or other container.
Coffee roasting thus requires careful control of numerous factors which influence the taste and appearance of the product. Because the coffee roasting business is competitive, economic factors such as capital costs, energy costs and coffee loss during the process are of great significance. Waste products discharged from coffee roasting processes can be a source of pollution. It is important to minimize such pollution while still maintaining an economical process and without comporomising the quality of the finished product.
All of the aforementioned factors together make coffee roasting a complex and difficult process. A vast number of methods and apparatus for roasting coffee have been proposed. Most commercial coffee roasting processes currently in use are performed at atmospheric pressure by contacting the coffee with hot gases such as a hot inert gas, typically nitrogen. The incoming gas heats the coffee beans whereas the outlet gas carries off waste products such as chaff and gases evolved in roasting. Traditional roasting methods can achieve only limited rates of heat transfer to the beans, and cannot provide full control of the bean time and temperature history. Further, traditional roasting methods and apparatus require significant effort and expense to minimize pollution.
Various proposals have been advanced for high-pressure roasting systems. Notably, numerous patents issued to Horace L. Smith Jr. describe batch or continuous systems for pressure-roasting of coffee in rolling fluidized beds or spouted beds. A xe2x80x9cfluidized bedxe2x80x9d system directs a gas or other fluid upwardly through a mass of particulates such as coffee beans, so that the particulates are held suspended in the rising fluid. Ideally, the upward flow is nearly uniform in all regions of the bed. A xe2x80x9cspouted bedxe2x80x9d system utilizes upward flow of the gas or other fluid concentrated at a few locations within the bed. The particles move upwardly at these locations and downwardly at other locations in the bed. Most of the Smith patents call for use of pressurized, low-oxygen-content gas circulating in a closed loop through: a heater, a bed of roasting coffee in a heavy-walled, cylindrical chamber and a cyclonic separator. The cyclonic separator removes small particles, commonly referred to as xe2x80x9cchaffxe2x80x9d from the gas. Some of the Smith patents use gas pressures up to 300 psig (2.1 MPa gauge). In a specific example, Robustas were roasted at 150 psig to improve their flavor. The roasting gas was heated by indirect contact with either a high-temperature, heatexchange fluid or hot gases produced in a fuel-fired furnace. To remove undesirable aromas, improve coffee flavor or puff roasting coffee, part of the roasting gas was bled off in some cases and replaced by inert gas produced by combustion of fuel. Certain Smith patents suggest that undesirable aromas also could be removed by condensation or scrubbing. Processes and methods disclosed in these patents suffer from certain fundamental limitations relating to the physical characteristics of the beds. If the gas velocity through the bed is increased, the fluidization becomes excessive. Beans can be entrained with the gas and carried out of the roaster into the remainder of the system. Moreover, the proper operation of the beds depends strongly on the depth of beans in the bed. Circulation of beans within the bed is suppressed if the bed is too shallow, whereas slugging and erratic spouting occur if the bed is too deep. Moreover, the Smith patents do not provide particularly precise control or repeatability in the process, inasmuch as these patents rely principally on control of gas inlet temperature to the roaster together with end-of-roast temperature or color measurements to indicate when the roasting procedure is complete.
Thus, despite these and other efforts in the art, there has been a significant need in the art for improvements in coffee roasting methods and apparatus.
The present invention addresses these needs.
One aspect of the invention provides methods of roasting coffee. The preferred methods according to this aspect of the invention include the steps of placing a charge of beans into a roasting chamber having a top and bottom, and directing a hot inlet gas through the beans from adjacent the bottom of said chamber and out of said chamber adjacent said top of said chamber to thereby form a fluidized or suspended bed of beans in said chamber and supply heat to the beans, whereby an exhaust gas including at least some of the inlet gas together with roasting byproducts will be discharged from said chamber. Methods according to this aspect of the invention most preferably include the step of trapping beans which are carried upward in the chamber by the flowing gas using a screen disposed adjacent said top of said chamber. The methods also most preferably include the step of moving a shutter in proximity to the screen so as to momentarily block gas flow through different sectors of the screen. Thus, beans held in engagement with the screen by the flowing gas will be released from the screen in each sector when gas flow through such sector is blocked.
The preferred methods according to this aspect of the invention can employ extraordinarily high gas flow rates through the chamber while still maintaining well-controlled patterns of bean circulation within the chamber, without losing beans in the exhaust gas and without packing the beans into a solid slug against the screen. For example, the mean velocity of the gas flowing in the chamber can be on the order of 0.5 meters/sec or more, and most typically about 0.5-2.5 meters/sec. Gas velocities of about 0.5-1.7 meters/sec are more preferred at relatively low inlet air temperatures of about 260xc2x0 F., whereas velocities up to about 2.5 meters/sec are more preferred at higher inlet air temperatures. Most preferably, the gas within the roasting chamber is maintained under a superatmospheric pressure, typically about 50-about 300 psig, i.e., about 0.35 to about 2.1 MPa gauge. The preferred methods according to this aspect of the invention can provide extraordinarily high rates of heat transfer to the beans, while maintaining excellent uniformity throughout the charge of beans and precise control of process conditions. The high rates of heat transfer available in the most preferred processes according to this aspect of the invention lead to several significant advantages, including high throughput in apparatus of reasonable size, as well as the ability to achieve temperature-versus-time profiles which are different from the temperature-versus-time profiles normally employed. Moreover, these conditions can be achieved with reasonable consumption of energy for pumping gas through the chamber.
The hot inlet gas, and hence the exhaust gas, typically consists predominantly of non-reactive gas components which are substantially non-reactive with said beans, such as nitrogen and carbon dioxide. Most preferably, at least some of the exhaust gas is reheated by passing said gas through a heater that generates heat and transfers it to said exhaust gas through an impermeable wall, and then passed back into the roasting chamber as inlet gas. Desirably, a charge of gas circulates through a substantially closed gas circulation system including said chamber so that said charge of gas is substantially retained within said circulation system during the process. The process desirably is performed using multiple charges of coffee beans, and hence includes the steps discharging the charge of beans from the chamber and reloading the chamber with a new charge of beans while substantially retaining the charge of gas within said circulation system. These steps are repeated cyclically so as to roast a series of charges of beans while substantially retaining the charge of gas within said circulation system.
In particularly preferred methods according to this aspect of the invention, the circulation system includes a cooler, and the method further includes the step of cooling each charge of beans within the roasting chamber prior to discharging that charge of beans from the chamber by circulating a portion of the charge of gas through a first portion of the circulation system including the cooler and the chamber. Preferably, this step is performed without circulating the first portion of the charge of gas through the heater. The same extraordinarily rapid rates of heat transfer which prevail during the roasting steps can be achieved during cooling, and hence the beans can be quenched effectively by the circulating cooled gas. A second portion of the charge of gas may continue to circulate through the remainder of the circulation system, including the heater, during this phase of the process.
The methods according to this aspect of the invention may further include the step of venting a selected portion of the charge of gas. Desirably, the venting procedure is performed so as to vent little or no gas during roasting of one or more early charges until the gas within said circulation system attains a desired level of volatile bean products and then vent more gas during roasting of one or more later charges so as to maintain the level of volatile bean products within the gas substantially constant.
A further aspect of the invention provides methods of roasting coffee beans which include the steps of supplying heat to the beans while the beans are disposed in an enclosed roasting chamber while directing a gas through the chamber so that an exhaust gas containing roasting byproducts including solid chaff and water vapor evolved from the beans is discharged from said chamber, venting at least a portion of the exhaust gas through a recovery device so as to remove chaff from the vented exhaust gas and condensing water vapor from the vented exhaust gas so that the condensed water wets at least some of the removed chaff. In one preferred method according to this aspect of the invention, the step of venting the exhaust gas includes the step of directing the vented exhaust gas through a cyclone having cooled walls so that said cyclone removes chaff from the exhaust gases and condenses water vapor from the exhaust gas. Removal of water along with at least some of the chaff simplifies the task of controlling the chaff ejected from the system to avoid pollution.
Yet another aspect of the invention provides methods of roasting coffee beans including the step of supplying heat to the beans while the beans are disposed in an enclosed roasting chamber by burning a fuel in a burner outside of said chamber so that the products of combustion generated in said burner are isolated from said beans, venting an exhaust gas from said chamber including roasting byproducts, and recycling at least a portion of the vented exhaust gas into the burner to thereby provide a mixture of fuel, air and exhaust gas. For example, at least some of the vented exhaust gas may be incorporated into the fuel stream prior to admixture with the air stream. Preferred methods according to this aspect of the invention include the step of monitoring the composition of combustion products produced by the burner and controlling the composition of the mixture responsive to said monitoring. Typically, the exhaust gas includes roasting byproducts in a substantially non-reactive gas such as nitrogen or carbon dioxide. The recycling and controlling steps desirably are performed so as to maintain the mixture at about 8% oxygen content. This provides for stable combustion but minimizes production of nitrogen oxides in the burner.
A still further aspect of the invention provides methods of roasting coffee beans with enhanced control. The preferred methods according to this aspect of the invention include the steps of providing coffee beans in a roasting chamber and directing a heated gas into said chamber and through said chamber so that the heated gas contacts the beans; preselecting a desired roasting bean temperature versus time profile; monitoring either or both of (1) the temperature of the beans; and (2) a set of parameters sufficient to determine the enthalpies and mass flow rates of the inlet gas and exhaust gas; and adjusting the condition of the inlet gas directed into the chamber responsive to the results of said monitoring step during the process to minimize deviations between the determined and desired bean temperature versus time profiles. The monitoring step may include the step of monitoring the outlet temperature of gas leaving the chamber. As the outlet temperature will vary closely with the temperature of the beans, the temperature of the beans can be monitored effectively in this manner. Most desirably, the monitoring step includes the step of monitoring the inlet and outlet pressures and temperatures and determining the amount of heat delivered to the beans as a function of time from these pressures and temperatures. As the actual heat transfer to the beans can be monitored during the process, the temperature versus time conditions can be controlled and matched to a predetermined profile. The method may further include the step of detecting when the amount of heat delivered to the beans equals the desired amount of heat necessary for roasting and halting roasting when such condition occurs. Desirably, the step of directing gas through the chamber includes the step of circulating at least a portion of the gas in a substantially closed circulation system from the chamber through a condenser and a heater, and the step of adjusting the inlet gas conditions includes the step of adjusting heat removal from the gas at said condenser.
The preferred methods according to the present invention include combinations of the foregoing aspects, and particularly preferred methods include all of these aspects of the invention. As will be further explained below, the various aspects of the invention interact with one another. Merely by way of example, control of bean temperature and time profiles is especially effective using the preferred roasting methods with high heat transfer rates as discussed above.
Still another aspect of the present invention provides a roaster for roasting coffee beans and other particulate vegetable materials. A roaster according to this aspect of the invention includes a structure defining an enclosed roasting chamber having a top and a bottom, and one or more bean transfer openings to permit the introduction of beans into the roasting chamber and withdrawal of beans from the roasting chamber. The roaster further includes a gas inlet communicating with the roasting chamber adjacent the bottom of the chamber to direct gas through the beans, as well as a top screen disposed adjacent the top of said roasting chamber, the top screen including a plurality of openings no larger than the size of the beans. A gas outlet communicates with the roasting chamber above the top screen so that gas directed through the beans will pass through the top screen before passing through the gas outlet.
Most preferably, the roaster further includes a shutter mounted for movement over a range of positions in proximity to said top screen to cut off the flow of gas through a shifting sector of the top screen as the shutter moves across the screen, whereby beans engaged on the top screen at such sector will fall back into the chamber, away from the top screen. Preferably, the shutter is mounted above the top screen. Roasters according to this aspect of the invention can provide advantages similar to those discussed above in connection with the methods, including high rates of heat transfer to and from the beans.
The top screen may be in the form of a surface of revolution about a central axis, and the roaster may further include a shaft mounted in the chamber for rotation about the central axis, the shutter being mounted to the shaft so that the shutter can be moved across said top screen be rotating the shaft. An agitator may be mounted to the shaft beneath the top screen for agitating beans in the chamber.
The roaster may further include a bottom screen having a plurality of openings disposed adjacent the bottom of said chamber, so that the top and bottom screens bound a central region of the chamber for holding beans to be roasted. In this case, the gas inlet desirably communicates with the chamber beneath the bottom screen. The bottom screen may have a bean outlet aperture coaxial with the shaft, the shaft having a hub mounted thereon below the shutter, the shaft being axially movable between an operating position in which the hub occludes the bean outlet aperture and a discharge position in which the hub does not occlude the bean outlet aperture.
The bottom screen desirably has a sloping surface extending from a highest portion to a lowest portion and the bean outlet aperture desirably extends through the bottom screen at the lowest portion. For example, the bottom screen may be generally conical and may have its lowest portions adjacent the tip of the cone, the bean outlet aperture being disposed at the tip of the cone. Desirably, the open area of the screen per unit of horizontal projected area of the bottom screen is greater in lowest portion of the screen than in the highest portion of the screen. This provides less resistance to gas flow in those regions of the screen aligned with the thicker portions of the bean mass in the chamber, and helps to equalize the flow throughout the chamber.
Yet another aspect of the present invention provides a pressure roasting system having a substantially closed circulation system including a roasting chamber for retaining a charge of beans to be roasted, a heater and at least one circulation blower connected to one another for circulating a gas under pressure in the circulation system through the roaster and heater. The system according to this aspect of the invention desirably also includes a pressure storage tank for holding gas at a pressure slightly above the maximum pressure used in the circulation system, a pressure release tank and a compressor connected between the pressure release tank and the pressure storage tank for transferring gas from the pressure release tank to the pressure storage tank to thereby maintain the pressure release tank at a pressure substantially lower than the pressure used in the circulation system. One or more selectively-operable pressure release valves may be provided for venting gas from the circulation system to the pressure release tank, along with one or more selectively-operable gas charging valves for transferring gas from the pressure storage tank to the circulation system. As further discussed below, this arrangement provides for rapid pressure release from the circulation system, which can be used, for example to control and promote puffing of the coffee beans, without substantial loss of inert gas and aroma constituents and without the atmospheric pollution problems associated with rapid venting of large amounts of gas from a roasting system.
A roasting system according to a further aspect of the invention has a substantially closed circulation system including a roasting chamber for retaining a charge of beans to be roasted, a heater and at least one circulation blower connected to one another for circulating a gas in the circulation system through the roaster and heater. The system further includes a chaff separator, the chaff separator including wall structure defining a separation chamber connected in the circulation system so that gas passing though the circulation system will pass through the separation chamber, and means for physically separating chaff from gas passing in the separation chamber. The system further includes means for cooling gas as it passes through the chaff separation chamber to thereby condense water vapor from the gas in the separation chamber a waste outlet communicating with the separation chamber for discharging water and chaff. The chaff separator may be is a cyclonic separator, and the means for physically separating may include means for directing gas passing through the separation chamber to flow in a cyclonic pattern. Desirably, the means for cooling gas includes means for cooling the wall structure of the separation chamber. The system according to this aspect of the invention may further include a scraper mounted in the separation chamber for mechanically dislodging chaff from the wall structure of the separation chamber, and may also include a discharge auger mounted in the separation chamber for forcing chaff and waste out of the separation chamber through the waste outlet.
These and other objects, features and advantages of the invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.